You have already deployed OWA signatures to on-premise users but OWA signatures are not deployed to Office 365 users.

Solution:

Using multiple Signature Deployment Policies, it is possible to configure which OWA signatures should be deployed to Office 365 or the on-premise Microsoft Exchange server.

A Signature Deployment Policy will create signatures for users that are members of an Organizational Unit. Therefore the on-premise users and Office 365 users would be required to reside in different OU’s.

Office 365 users that apply to an OWA signature policy will be deployed the OWA signature after configuration has saved. Click the Status link on the right hand side of the Exclaimer console to view the event logs to confirm the Office 365 user has been deployed an OWA signature successfully.